Human transmissible spongiform encephalopathies (TSEs) manifest in sporadic, familial and transmissible forms, each of which are thought to arise through the abnormal metabolism of the cellular prion protein (PrPc). Multiple cases of iatrogenic CJD and new variant CJD have raised fears over the risk for horizontal transmission of infectious prions, either via the blood supply or through ingestion of contaminated foods. Prolonged incubation periods after infection make an accurate assessment of these dangers difficult to determine. There is therefore an urgent need for a reliable, rapid and sensitive diagnostic test to identify prion-contaminated tissues. PrPSc, a conformational isomer of PrPc, remains the only consistent correlate for prion disease and infectivity. Here we propose to develop a conformation-dependent immunoassay in which PrP-specific monoclonal antibodies will be used to screen for PrPSc in tissue samples. We aim to take existing antibodies, as well as novel antibodies generated in the course of this study, and to apply sequential rounds of mutagenesis and phage selection to evolve molecules binding PrP with ultra-high affinities. Preliminary studies using a model assay system suggest that antibodies binding PrP with picomolar affinities will be capable of detecting PrPSc at levels equivalent to a single ID50 of prion infectivity per ml of tissue.